Chemical lasers require a gaseous fuel supply in which the fuel is provided in a vaporized form. It is also generally advisable that the fuel supply system associated with a chemical laser be relatively light. In order to reduce the weight of the fuel supply system, the fuel is therefore generally stored as a liquid. Since the fuel is stored as a liquid, the fuel supply system must be capable of providing the heat required for vaporization of the liquid in real time, that is, as the chemical laser is activated and requires a supply of gaseous fuel. The chemical laser operates for very brief periods, during which time the fuel heat of vaporization must be provided at a high rate. The time between laser operations can be much longer; during this period the thermal energy required for a subsequent laser operation can be accumulated at a much lower rate.
In order to vaporize the fuel in real time, the fuel supply system includes heat transfer elements that are submerged within a generally substantial volume of liquid fuel. Prior to activation of the chemical laser, the heat transfer elements are heated so as to store the energy to be subsequently used to vaporize some of the fuel that has been delivered to the chemical laser. In order to vaporize the fuel effectively, however, the heat transfer elements must generally remain submerged within the liquid fuel with the volume of the liquid fuel required to submerge the heat transfer elements generally comprising a significant portion of the total fluid inventory and, as such, the weight of the system. Further, the heat transfer elements must contain sufficient heat and have sufficient surface area to provide the fuel heat of vaporization for the desired time, at the desired rate and at the desired conditions of temperature and pressure.
One typical type of heat transfer element for fuel supply systems associated with chemical lasers are pebble bed heaters in which a bed of balls serve as the heat transfer elements and, accordingly, provide the thermal mass for the liquid vaporization. In a pebble bed heater, the minimum volume of liquid fuel that is required to submerge the balls in order to provide adequate heat transfer from the balls to the liquid for vaporization purposes is generally at least about 35% of the aggregate volume of the balls. As such, the liquid that is normally required to submerge the balls can create a potentially large fluid inventory which, in turn, adds to the weight of the fuel supply system. While the liquid required to submerge the balls or other heat transfer elements may substantially add to the weight of the fuel supply system, this volume of the liquid is generally not used as fuel for the laser since the balls must remain submerged within the liquid in order to provide effective heat transfer from the balls to the vaporizing liquid.
Accordingly, it would be desirable for a fuel supply system, such as the fuel supply system associated with a chemical laser, to provide sufficient heat storage and delivery rate for the vaporization process while reducing the weight of the fuel supply system, such as by reducing the liquid inventory required for heat transfer purposes.